Going Green for Life

Jan. 21, 2010
Why hasn't Earth warmed as much as expected? New report explores reasons
*By **Vince Stricherz*
News and Information

Earth has warmed much less than expected during the industrial era based
on current best estimates of Earth's "climate sensitivity" -- the amount
of global temperature increase expected in response to a given rise in
atmospheric concentrations of carbon dioxide.

In a study published online on Jan. 19 in the /Journal of Climate/,
Stephen Schwartz of Brookhaven National Laboratory, Robert Charlson of
the UW and colleagues examine the reasons for this discrepancy.

According to current best estimates of climate sensitivity, the amount
of carbon dioxide and other heat-trapping gases added to Earth's
atmosphere since humanity began burning fossil fuels on a significant
scale during the industrial period would be expected to result in a mean
global temperature rise of 3.8 degrees Fahrenheit. That is much more
than the 1.4 degrees F. increase that has been observed for this time span.

"The data show that either we have 40 years of emissions left before the
atmosphere can't absorb any more carbon dioxide, or we're already past
the point of no return. In other words, the uncertainty rate is
unacceptably high," said Charlson, a UW atmospheric sciences professor
who in the 1960s invented a device called the integrating nephelometer
to measure atmospheric haze particles, producing data that is still used
in climate models today.

The new analysis attributes the reasons for the discrepancy between
projected and actual temperature increase to a possible mix of two major
factors: Earth's climate may be less sensitive to rising greenhouse
gases than currently assumed and/or reflection of sunlight by haze
particles in the atmosphere may be offsetting some of the expected warming.

"Because of present uncertainties in climate sensitivity and the
enhanced reflectivity of haze particles," said Schwartz, "it is
impossible to accurately assign weights to the relative contributions of
these two factors. This has major implications for understanding Earth's
climate and how the world will meet its future energy needs."

A third possible reason for the lower-than-expected increase of Earth's
temperature over the industrial period is the slow response of
temperature to the warming influence of heat-trapping gases.

"This is much like the lag time you experience when heating a pot of
water on a stove," said Schwartz. Based on calculations using
measurements of the increase in ocean heat content over the past 50
years, however, the present study found the role of so-called thermal
lag to be minor.

A key question facing policymakers is how much additional carbon dioxide
and other heat-trapping gases can be introduced into the atmosphere,
beyond what is already present, without committing the planet to a
dangerous level of human interference with the climate system. Many
scientists and policymakers consider the threshold for such dangerous
interference to be an increase in global temperature of 3.6 degrees F
above the preindustrial level, although no single threshold would
encompass all effects.

The paper describes three scenarios. If Earth's climate sensitivity is
at the /low end /of current estimates as given by the Intergovernmental
Panel on Climate Change, then the total maximum future emissions of
heat-trapping gases so as not to exceed the 3.6-degree threshold would
correspond to about 35 years of present annual emissions of carbon
dioxide from fossil-fuel combustion. A climate sensitivity consistent
with the present best estimate would mean that no more heat-trapping
gases can be added to the atmosphere without committing the planet to
exceeding the threshold. And if the sensitivity is at the /high end /of
current estimates, present atmospheric concentrations of heat-trapping
gases are such that the planet is already committed to warming that
substantially exceeds the 3.6-degree threshold.

The authors emphasize the need to quantify the influences of haze
particles to narrow the uncertainty in Earth's climate sensitivity. The
task is much more difficult than quantifying the influences of
heat-trapping gases, said Charlson, who likens the focus on
heat-trapping gases to "looking for the lost key under the lamppost."

Schwartz observes that formulating energy policy with the present
uncertainty in climate sensitivity is like navigating a large ship in
perilous waters without charts. "We know we have to change the course of
this ship, and we know the direction of the change, but we don't know
how much we need to change the course or how soon we have to do it."

"These results do not in any way reduce or remove the need for solid
action now to move toward a zero-carbon dioxide-emission economy. The
results tell us that doing our utmost now might work very well if the
most optimistic values of sensitivity are real, but that it is possible
that nothing will work no matter how hard we try," Charlson said.

"If we do not reduce uncertainties, we will be in the same boat 10 or 20
years from now as we are today," he said.

Schwartz and Charlson coauthored the paper with Ralph Kahn, NASA Goddard
Space Flight Center in Maryland; John Ogren, NOAA Earth System Research
Laboratory in Colorado; and Henning Rodhe, Stockholm University.

The research was funded by the U.S. Department of Energy Office of Science.